The grating is a key element widely used as a high efficient dispersive optical element, and it plays an important role in the optical instrumentation.
As well known, the direction of the main diffraction peak for a single slit is both the geometric optical propagation direction of the light and the direction of the zero order for the multi-slit grating. It is in the direction of the main diffraction peak for the single slit, where the light with various wavelengths concentrate. In practical application, it is tended that the light energy is concentrated on a specific order as far as possible. Therefore, it is necessary to fabricate a groove determined by a calculation, such that the direction of the main diffraction peak for a single slit (or the geometric optical propagation direction of light) is consistent with the direction of the predetermined diffraction order for the grating. In this case, most of the light energy may be concentrated on the predetermined diffraction order. The phenomenon is referred to as blaze, and the grating is referred to as a blazed grating. The diffraction efficiency of the grating is greatly improved due to the blaze.
Although the blazed grating has a lot of advantages, it is difficult to keep obtaining high diffraction efficiency in a broad waveband from an ultraviolet band to an infrared band. Therefore, the holographic bi-brazed grating realizes higher and uniform diffraction efficiency in the broad waveband. And the holographic bi-blazed grating has a broad market prospects due to its advantage of high efficiency in the broad waveband.
In Chinese patent application No. CN200910231737.3, a method for fabricating a holographic bi-blazed grating is disclosed. In the method, a grating with a blaze angle A is fabricated on a substrate firstly, then a region A is shielded, and then a grating with a blaze angle B is fabricated on a region B. Holographic ion beam etching is adopted in fabricating the two gratings, which includes: fabricating a photoresist grating mask on the surface, etching the mask with an tilted-ion beam to fabricate a triangle blazed grating on the substrate. The two blaze angles are fabricated by controlling the thicknesses of the photoresists on both of the regions A and B.
However, in the above method, the processes of coating photoresist and lithography are performed twice to form the two blaze angles of the grating. Besides, fabrication of the blaze angles A and B depends on a groove depth of the photoresist grating. It is difficult to precisely control a duty cycle, a groove shape and a depth of the grating which is formed by performing the lithography on the photoresist. In addition, in the case that the tilted-ion beam etching is performed, a difference between the materials of the photoresist and the substrate results in inconsistent etching rates, hence there is an error between the blaze angle of the blazed grating fabricated and an desired blaze angle, and it is difficult to precisely control the blaze angle.
Therefore, it is necessary to seek for a new method for fabricating a holographic bi-blazed grating to solve the above issue.